When a microscope is used to focus into a biological specimen, spherical aberrations caused by a refractive-index mismatch can occur that vary depending on the specimen depth. These aberrations lead to a reduction in the resolution and contrast. Moreover, together with the working distance of the objective as well as the absorption and scatter that occur in the specimen, they limit the maximum depth of penetration into the specimen.
For purposes correcting spherical aberrations, there are objectives with a correction ring that moves a lens group within the objective along the optical axis. The lens group that serves for the correction has to be taken into account during the optical design of the objective. As a rule, this can only be achieved with a great deal of mechanical effort. In particular, it is not possible to retrofit an objective with such a correction unit.
For purposes of correcting spherical aberrations, European patent application EP 0 859 259 A2 proposes arranging adapter pieces between the objective and the tube lens, said adapter pieces consisting of lenses that are cemented together and that can introduce or compensate for a variable spherical aberration. However, the proposed arrangement directly above the objective generally changes the parfocal length of the entire system. Moreover, the above-mentioned adapter pieces necessarily have to be configured for the specific objective because of the different pupil sizes of objectives having a different focal length.
U.S. Pat. No. 8,659,827 B2 discloses an adjustable afocal system that is arranged in the infinite beam path and that can introduce or compensate for spherical aberrations. Since this afocal system is arranged in an area of the beam path in which light is propagated simultaneously to various field points, the system has to be corrected for a complete field of view. This greatly restricts the correction effect that can be achieved.
International patent application WO 2012/025349 A1 shows a correction system that has freeform surfaces that can be moved laterally with respect to each other in order to compensate for spherical aberrations. The freeform surfaces needed for this correction system, however, have a complicated design and are thus expensive to produce.